1. Field of the Invention
The present invention relates to an electronic fuel injector for supplying a fuel spray to an automobile internal combustion engine, and more particularly to an electronic fuel injector suitable for use in the type of injector including a swirler.
2. Description of the Related Art
Conventional electronic fuel injectors for use with gasoline fuel, by way of example, are generally divided into two types, i.e., the ball type in which a movable part has a ball-shaped fore end as disclosed in Japanese Unexamined Patent Application Publication No. 1-310165, and the pintle type in which a movable part has a triangular fore end. Those two types are however substantially similar in both structure and function. More specifically, any type of electronic fuel injector comprises a stator core, an electromagnetic coil arranged concentric with the stator core, a casing made of a magnetic material and containing the stator core and the electromagnetic coil, a movable part having a valve member provided at its fore end, a stopper for stopping movement of the movable part, a valve seat arranged in an opposed relation to the stopper with the movable part interposed therebetween, and a spring engaging with one end of the movable part to press the movable part against the valve seat. When an electric current is supplied to the electromagnetic coil, a magnetic circuit is formed to generate an electromagnetic force. Upon the generated electromagnetic force overcoming a resilient force of the spring pressing the movable part, the valve member provided at the fore end of the movable part is moved away from the valve seat and the injector is opened. When the current is cut off, the valve member is moved into contact with the valve seat and the injector is closed.
In such a conventional electronic fuel injector, the movable part is vertically moved between the stopper and the valve seat. For the purpose of preventing the fore end of the movable part from wobbling laterally and ensuing the operation as smooth as possible, the electronic fuel injector includes a swirler put into frictional contact with the valve member provided at the fore end of the movable part. The swirler serves not only to guide the fore end of the movable part, but also to swirl fuel. Therefore, the swirler has such a complicated shape that grooves are formed as fuel passages in one surface of the swirler on the injection side so as to swirl the fuel. Because the swirler is put into frictional contact with the valve member under supply of high-pressure fuel and hence requires superior wear resistance, it is commonly manufactured by mechanically machining a material made of JIS-SUS440C, i.e., high-carbon and high-chromium martensitic stainless steel, with high precision, then quenching and tempering the material to harden it up to a level of about 60 HRC, and further finishing an inner cylindrical surface, etc. to remedy a deformation caused by heat treatment. As an alternative, in consideration of the fact that the grooves formed as fuel passages in the swirler have a complicated shape and mechanical machining of the swirler requires the increased number of steps and a longer working time, the swirler is manufactured by MIM (metal injection molding) using a powder of SUS440C, or by powder sintering using a powder of (Fe—Ni based) permalloy having low hardness and good fluidity when a high level of wear resistance is not required.
Of the swirlers used in conventional fuel injectors, one manufactured by mechanical machining of SUS440C has problems in that the number of machining steps is increased and the life of a cutter is shortened, because a SUS440C material, which is hard to machine, must be machined into the swirler including grooves of a complicated shape formed as fuel passages and having an inner cylindrical surface finished into a desired inner diameter with high precision, before the machined swirler is subjected to heat treatment. Further, when burrs and/or buckles, for example, remain in the swirler after the mechanical machining and the finishing, worn-out dust is generated upon wear of the swirler that is put into frictional contact with a valve member provided on a movable part. The generated worn-out dust acts as an abrasive and accelerates the wear of the swirler. If the worn-out dust is fixedly caught in a fuel sheet formed on a valve seat between itself and the valve member, there arises a risk of fuel leakage. Also, the swirler manufactured by MIM has problems in that a post-process is needed due to a difficulty in achieving the required accuracy by MIM alone, and hence the production cost is pushed up. Further, the swirler manufactured by powder sintering has problems in that because the used powdery raw material is relatively soft, satisfactory dimensional accuracy can be obtained, but wear resistance is poor.
The above-mentioned problems are more significant particularly in a direct-injection combustion system in which an increased surface pressure occurs between the swirler and the counterpart, i.e., the valve member provided on the movable part and put into frictional contact with the swirler. More specifically, the fuel pressure rises to a level of 7 to 15 MPa in the direct-injection combustion system, and a much higher surface pressure than that in an ordinary combustion system is applied to between the swirler and the valve member provided on the movable part. This brings the swirler into an abrasively worn state in which both friction wear and impact wear occur, whereby worn-out dust is generated. The generated worn-out dust acts as an abrasive and accelerates the both types of wear of the swirler. In the case of mechanically machining a SUS440C material that is used in many swirlers of conventional electronic fuel injectors, the swirler has hardness as high as about 60 HRC as a result of quenching and tempering, and hence a relatively good level of wear resistance is obtained. However, because the swirler and the counterpart, i.e., the valve member provided on the movable part, are made of the same material, inter-molecular coupling tends to easily occur due to the friction wear, and hence the swirler having such a material combination cannot be said as being optimum. Further, not a few burrs and/or buckles occur in the swirler with the mechanical machining thereof into a complicated shape, and they must be removed in a post-process such as barrel polishing. The burrs and/or buckles remaining in spite of the post-process generate abrasive dust in many cases.